Why was Clark Kent unable to hold the falling steel structure of the oil rig?

The Clark Kent at that period of time was not yet Superman. He was aware of his immense strength, hyper-senses and laser eyes. He certainly did not know he could fly, though he could jump quite high. Simply put, he was not aware of his true strength.

Also, he mostly lived a secluded and mysterious life. Not being open about his powers was a choice which he lived with. When he tried to stop the Steel Tower from falling down, it was only for a few moments during which the people could safely escape in the helicopter. He physically stopped the tower from falling by standing in between and taking its weight.

However, due to the way the steel tower was positioned, it's foundation continued to topple off from the other end, resulting in the tower losing its balance, and falling in the sea. Clark stopped the tower from falling, but the structure finally toppled off due to imbalance.

There's a scene when Jor-El meets Clark in which he says (I'm afraid I don't know the exact quote). "If you keep pushing yourself who knows how strong you can become".

At this point in the film Clark had been living an ordinary life, avoiding conflict and not testing his powers. It's only once he starts to become Superman that his strength increases even further and he gains other abilities (flight for example).

In short, when he's on the oil rig although he's far stronger than any human he's not Superman strong!